A sintered R-T-B based magnet, including an R2T14B type compound as a main phase, is known as a permanent magnet with the highest performance, and has been used in various types of motors such as a voice coil motor (VCM) for a hard disk drive and a motor for a hybrid car and in numerous types of consumer electronic appliances.
As a sintered R-T-B based magnet loses its coercivity at high temperatures, such a magnet will cause an irreversible flux loss. For that reason, when used in a motor, for example, the magnet should maintain coercivity that is high enough even at elevated temperatures to minimize the irreversible flux loss.
It is known that if R in the R2T14B type compound phase is replaced with a heavy rare-earth element RH (which may be Dy and/or Tb), the coercivity of a sintered R-T-B based magnet will increase. It is effective to add a lot of such a heavy rare-earth element RH to the sintered R-T-B based magnet to achieve high coercivity at a high temperature.
However, if the light rare-earth element RL (which may be at least one of Nd and Pr) is replaced with the heavy rare-earth element RH as R in a sintered R-T-B based magnet, the coercivity certainly increases but the remanence decreases instead. Furthermore, as the heavy rare-earth element RH is one of rare natural resources, its use should be cut down.
Patent Document No. 1 discloses a technique for increasing the coercivity of a magnet. According to that technique, powder of an oxide, a fluoride, or an oxyfluoride of a heavy rare-earth element RH is put on the surface of a sintered magnet, and the sintered magnet is subjected to a heat treatment at a temperature that is equal to or lower than the sintering temperature of that sintered magnet in either a vacuum or an inert gas, thereby diffusing the heavy rare-earth element RH from the surface of the sintered magnet and increasing the coercivity of the magnet.
According to Patent Document No. 1, such a powder can be put on the surface of sintered magnet (as a powder processing method) by immersing the sintered magnet in a slurry, in which a fine powder of a heavy rare-earth element compound, including one or two or more of an oxide, a fluoride, and an oxy-fluoride, is dispersed in water or an organic solvent, drying the sintered magnet with hot air or in a vacuum, and then subjecting the magnet to a heat treatment so that the heavy rare-earth element RH is introduced through the surface of the magnet. According to Patent Document No. 1, a compound including a fluoride, in particular, can be absorbed into the magnet highly efficiently and the coercivity can be increased very effectively.
On the other hand, according to Patent Document No. 2, a sintered R-T-B based magnet is buried in an oxide or fluoride powder of a heavy rare-earth element RH and then subjected to a heat treatment at 500° C. to 1000° C. for 10 minutes to 8 hours in Ar or He, thereby forming an insulating layer in a surface region of the sintered magnet.